The present invention relates to the production of optically isotropic liquid crystal materials and the structures of a liquid crystal display panel and a liquid crystal display device which use the liquid crystal materials.
Owing to the recent advance in techniques for producing a liquid crystal panel, liquid crystal display elements have come to be used in a display for television though Braun tubes have heretofore been mainly used therein. As to the liquid crystal display elements, twisted nematic (TN) display method has heretofore been known but has been required to be improved in contrast, view angle characteristics and response characteristics. Particularly when the liquid crystal display elements are used in televisions, they are greatly inferior to Braun tubes in the characteristic described above. Therefore, there has been a desire for the improvement of these characteristics. As a display method for the liquid crystal display elements for improving the above-mentioned contrast and view angle characteristics, there are known, for example, in-plane switching (transverse electric field) display method (hereinafter referred to as “IPS method”) and multi-domain vertical alignment display method (hereinafter referred to as “VA method”). These methods permit great improvement in view angle and contrast as compared with TN method.
However, in IPS and VA methods, a liquid crystal layer is an optically uniaxial medium, so that its transmittance becomes dependent on view angle when the liquid crystal layer is used as it is. In addition, as described in the non-patent document 1 described hereinafter, nematic liquid crystal materials shows light scattering caused by the thermal perturbation of molecules. In IPS and VA methods, since a black display is given when no voltage is applied, a lowering of contrast by light leakage due to the light scattering is unavoidable in principle even in the case of the black display. Problems including such optical anisotropy and light scattering are inherent in display devices using a nematic liquid crystal material.
On the other hand, in recent years, there have been known materials of liquid crystals having isotropy optically and three-dimensionally or two-dimensionally (hereinafter referred to as “optically isotropic liquid crystals”). The optically isotropic liquid crystals have the following property: the alignment of liquid crystal molecules in a liquid crystal layer is optically and three-dimensionally or two-dimensionally isotropic when no voltage is applied to the liquid crystal layer, and by the application of a voltage, birefringence is induced in the direction of application of the voltage. In the case of materials of optically isotropic liquid crystals reported in recent years, the following phases and molecules are optically isotropic. As phases having optical isotropy three-dimensionally, there are a smectic blue phase and a cholesteric blue phase. As molecules having optical isotropy two-dimensionally, there are bent core molecules. The bent core molecules are molecules of a liquid crystal compound subjected to vertical alignment with respect to a substrate and have optical isotropy in a plane of a liquid crystal layer when no voltage is applied. In addition to them, a cubic phase, a smectic Q phase, a micelle phase, a reverse micelle phase, a sponge phase, etc. are known as phases having optical isotropy.
The following non-patent document 2 and non-patent document 3 describe the extension of the temperature range of a blue phase which has been difficult to put to practical use in a device because of its very narrow temperature range. The following non-patent document 4 describes materials of optically isotropic liquid crystals and their properties, such as the optical biaxiality of bent core molecules. The following non-patent document 5 and non-patent document 6 describe display devices using optically isotropic liquid crystals. The following non-patent document 7 describes an electric field strength necessary to optically isotropic liquid crystals.
In addition, the following patent document 1 discloses specifically the electrode structure of a liquid crystal panel using optically isotropic liquid crystals, etc.    Patent Document 1: JP-A-2006-3840    Non-patent document 1: W. H. de Jew, “Properties of Liquid Crystal”, translated by Tsutomu Ishii and Shunsuke Kobayashi, pp. 90-94    Non-patent document 2: Harry J. Coles, Nature, Vol. 436, pp. 997-1000    Non-patent document 3: Atsushi Yoshizawa, et al., Journal of Materials Chemistry, Vol. 15, pp. 3285-3290, 2005    Non-patent document 4: Bharat R. Acharya et al. LIQUID CRYSTALS TODAY, VOL. 13, No. 1, 1-4, 2004    Non-patent document 5: Hiroshi Kikuchi, Advanced Materials, Vol. 17, pp. 96-98, 2005    Non-patent document 6: Hideo Takezoe et al., Japanese Journal of Applied Physics, Vol. 45, L pp. 282-284, 2006    Non-patent document 7: Physica B, Vol. 368, pp. 168-178, 2005
As described above, the properties of optically isotropic liquid crystals have been elucidated which are different from those of conventional liquid crystals, but the structure of a device using such a liquid crystal material has not sufficiently been investigated.